The present invention relates generally to a method for manufacturing cement using a mixture of finely ground raw materials including steel slag.
In the engineering and construction industry, the terms xe2x80x9ccementxe2x80x9d and xe2x80x9cPortland cementxe2x80x9d are generally used to describe hydraulic cement produced by burning a mixture of finely ground calcareous and argillaceous materials to form cement clinker. The burning process produces calcium silicates and calcium aluminates which chemically react with water to form a hard, stone-like mass. Clinker generally includes coarse granules and irregularly shaped nuggets often varying in size from two to three inches in diameter or greater. The resulting clinker is typically ground in a finishing mill to form fine dry cement powder. A small amount of gypsum is normally added during the clinker grinding process to retard setting of the resulting cement. The finely ground cement powder may be mixed with sand, coarse aggregate and water to produce mortars and concrete. Hydraulic cement or Portland cement generally includes tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite.
A wide variety of hydraulic cements are presently used in modern engineering and construction projects. Portland cements are normally made in five types. The characteristics and properties of these five types have been standardized on the basis of ASTM Standard Specification for Portland Cement(copyright) 150). The differences between Types I, II, III, IV and V are based on both chemical and physical requirements. Typically the chemical and physical composition of raw materials added to a kiln are adjusted to produce cement clinker have the appropriate chemical composition for the desired type of hydraulic cement.
U.S. Pat. No. 5,393,342 entitled Cement Composition and Method for the Preparation Thereof discloses a cement composition consisting of ground cement clinker, ground blast furnace slag and comminuted steel slags. The invention described in the ""342 Patent includes mixing ground steel slags at normal or elevated temperatures and pressures with ground cement clinker and ground, granulated blast furnace slags. The resulting mixture may be cooled if desired and comminuted to obtain a hydraulically hardenable cement composition. Example 2 of the ""342 Patent states:
xe2x80x9cThe steel slag powder was mixed with the raw materials customary for Portland cement clinker, the amount of steel slags added being about 25% of the total amount of blast furnace slags present in blast furnace slag cement, and treated at a temperature of about 1400xc2x0 C. in a cement furnace. After cooling, the steel-slag cement thus obtained was finely ground and mixed with ground, granulated blast furnace slags in order to obtain a cement composition.xe2x80x9d
U.S. Pat. No. 5,494,515 entitled Method and Apparatus for Using Blast-Furnace Slag in Cement Clinker Production and U.S. Pat. No. 5,421,880 entitled Method and Apparatus for Using Steel Slag in Cement Clinker Production disclose apparatus and procedures for using slag in the manufacture of cement.
Ferrolime, sometimes referred to as xe2x80x9clime slagxe2x80x9d, a byproduct from manufacturing steel, has previously been used in the manufacture of cement clinker with equipment such as shown and described in this patent application.
In accordance with teachings of the present invention, disadvantages and problems associated with previous methods for using steel slag to manufacture cement have been substantially reduced or eliminated. The present invention provides a method for manufacturing cement from a mixture of finely ground raw materials which includes steel slag.
Technical benefits of the present invention include allowing existing equipment associated with storing, mixing, grinding and processing raw materials used to manufacture cement to also incorporate finely ground steel slag as an integral part of the raw materials supplied to the feed end of a kiln. Mixing steel slag with other raw materials prior to placing the steel slag in a raw mill may minimize any tendency of the steel slag to cause excessive wear or damage to the raw mill. Additional equipment is not required to add steel slag to the kiln separate from other raw materials supplied to the feed end of the kiln. Adding steel slag to a kiln in accordance with teachings of the present invention eliminates the need to provide additional equipment such as a steel slag conveyor belt extending to the feed end of the kiln, a weigh feeder and perhaps a rotary feeder or an airlock. Depending upon the existing raw material storage system, addition of a steel slag storage bin may not be required. Processing steel slag in accordance with teachings of the present invention removes substantial amounts of any moisture present in the steel slag prior to adding the steel slag to the kiln. By removing moisture from finely ground steel slag prior to entering the kiln, heat losses are minimized and the production of non-combustible gases such as steam are reduced.
One aspect of the present invention includes mixing discrete quantities of steel slag with discrete quantities of other raw materials used to manufacture cement and comminuting the resulting raw material mixture in a raw mill. Exhaust gas from a kiln or waste heat from other processes associated with manufacturing cement is preferably supplied to the raw mill to remove any moisture contained in the steel slag and other raw materials. As a result of incorporating teachings of the present invention a dry powder including finely ground steel slag is produced by the raw mill. The finely ground steel slag will then react much more readily with other elements and chemical compounds contained in the raw material mixture when the dry powder is added to the feed end of a kiln. For example, steel slag often contains magnesium oxide (MgO) which melts at a relatively low temperature. Adding finely ground steel slag as an integral part of the raw material mixture supplied to the feed end of the kiln allows any magnesium oxide contained within the steel slag to more quickly melt and become liquid and assist with combining silica, aluminum and calcium to form chemical compounds associated with cement. Adding finely ground steel slag to the feed end of the kiln allows iron compounds, magnesium compounds and other compounds contained in the steel slag to start the liquification process earlier in the kiln which generally reduces overall energy consumption.
Further technical advantages of the present invention include the ability to adjust the quantity of steel slag and other raw materials supplied to the raw mill based on chemical composition of the steel slag and other raw material components and desired chemical characteristics of the resulting cement clinker. Prior to comminuting in the raw mill, steel slag generally has a substantially lower ratio of surface area to volume as compared to finely ground steel slag. Since thermal reactivity is a function of particle size, finely ground steel slag will generally be more thermally reactive when added to a kiln as compared to adding steel slag as received from a steel slag supplier to the same kiln.
The steel slag preferably includes a low concentration of iron which often reduces the need to add additional silica sand to the raw material mixture. For some applications, reducing the amount of silica sand required to produce the same amount of clinker, particularly silica sand containing quartzite, may reduce wear in the raw mill.
Mixing steel slag with other raw materials and placing the raw material mixture in a raw mill allows precise control of the ratio and proportions of the various raw material components. The type and characteristics of cement clinker which will be produced can be predetermined and adjustments made to the raw material mixture prior to supplying to a kiln. Accuracy of raw material calculations may be verified by analyzing the resulting clinker. The present invention avoids having to react to analytical variations in the clinker after it has been produced.